In recent years, tantalum or niobium powders (hereinafter referred to as “metal powders” as their generic name) obtained by reduction of potassium fluorotantalate or potassium fluoroniobate have been increasingly used in the field of capacitors. In order to produce a capacitor with a high performance, metal powders are required to have a high CV value, and therefore various studies have been carried out regarding metal powders.
Potassium fluorotantalate or potassium fluoroniobate is reduced to metal powders, when it is contacted with fumes such as alkaline metal or alkaline earth metal in an atmosphere with inert gas such as argon gas. These metal powders can be used as the electrodes of a capacitor after sintering. However, if potassium fluorotantalate crystals or potassium fluoroniobate crystals which are raw materials for metal powders contain too much oxygen, the amount consumed of alkaline metal or alkaline earth metal which is a reducer in a reduction reaction increases, and further the reduction reaction does not occur uniformly, and thus it is considered that the CV value cannot be improved.
CV value is a value used to express performance regarding the capacitance of a capacitor. The higher the CV value, the smaller capacitor with a larger capacitance can be produced. In recent years, a CV value per weight of metal powders (CV/g) has become large at a level of several ten thousands to several hundred thousands of CV/g, and it is still required to obtain a higher CV value. In order to obtain a higher CV value, it is important to reduce leakage current as well as manufacturing the fine size of metal powders. It is therefore desired to reduce impurities such as metal ions contained in metal powers to a minimum and to produce metal powders containing the lowest level of oxygen.
Nevertheless, tantalum and niobium are metals which have extremely high affinity for oxygen, and it is therefore considered that the reduction of the amount of oxygen contained in potassium fluorotantalate crystals or potassium fluoroniobate crystals would be an extremely difficult technique. Especially, potassium fluorotantalate crystals have a particularly high affinty for oxygen. The amount of oxygen contained in potassium fluorotantalate crystals which have been available on the market is over 3% by weight, and the amount of oxygen contained in potassium fluoroniobate crystals is over 3.5% by weight. The production of these crystals, the oxygen content of which is 2% or less by weight, has been considered to be impossible.
Against this backdrop, as raw materials for metal powders used for a capacitor, low oxygen-containing potassium fluorotantalate crystals or low oxygen-containing potassium fluoroniobate crystals which contain a low level of oxygen before a reduction reaction, have been required.